fix docker build (fix #56)
fix docker build (fix #56)

Summary: Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/61

Differential Revision: D17909051

Pulled By: ppwwyyxx

fbshipit-source-id: 76e8b59e4f0a855e1b81ab4418721628efe36332

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2